1. Field of the Invention
The invention relates an alkali-soluble resin for color filter, a photosensitive resin composition, and a color filter formed thereof which is used in a liquid crystal display. More particularly, the invention provides an alkali-soluble resin for color filter and a photosensitive resin composition thereof having good developability, high precision pattern linearity, and contour angle.
2. Description of Related Art
Due to having advantages of being light, thin, and low-power consuming, liquid crystal displays (LCDs) are applied in various products such as laptops, personal digital assistant devices, digital cameras, or desktops. However, such type of LCDs requires a color filter with high color reproduction range.
This color filter is obtained by forming hues of three colors, red (R), green (G), and blue (B) as stripe or mosaic patterns on a surface of a support such as a glass or a plastic sheet on which a black matrix (BM) is formed.
Up to date, several methods of manufacturing color filters have been proposed, and among them, the most well known method is a method for manufacturing the color filter through the usage of a negative-type photosensitive coloring composition. This negative-type photosensitive coloring composition consists of a dye, an acrylic resin, a photopolymerization initiator, and a reactive polymer. The negative-type photosensitive coloring composition is hardened by the following process. The photopolymerization initiator decomposed or activated by the ultraviolet irradiation starts polymerization because free radicals are formed. Thereafter, the ethylenic unsaturated group is activated by these free radicals to proceed radial polymerization. When using the negative-type photosensitive coloring composition to form the color filter, this negative-type photosensitive coloring composition is typically being coated on a substrate, irradiated by an ultraviolet light via a mask, and then developed to obtain a pattern. The pattern formed by the process mentioned above is heated to sticked on the substrate so as to form a pixel pattern. The cycle is repeated with the required colors to obtain a pattern of the colored coating. However, the repetition of the cycle will generate larger steps in the end of the BM and RGB pixels, and therefore the color displays unevenly resulting from the steps. In order to suppress the steps, a transparent resin layer (protective film) is introduced for the planarization of the color filter.
The protective film is required to have several traits, including the ability to protect the RGB colored layer, the thermal endurance for the liquid crystal filling process, and the hardness against the pressure force. A photosensitive curable resin composition with high cross linking density is developed to achieve the hardness mentioned above (with reference to Japanese Patent No. 5-78483). However, this photosensitive curable resin composition will go through contraction during the hardening process, and generates residual stress. This residual stress can easily cause defects in the high precision pattern linearity.
Although Japanese Patent No. 10-133372 mentioned that the defects in the high precision pattern linearity can be improved using a protective film composition containing epoxy compound, defects of poor developability and contour angle in high precision still exist.
Moreover, as the personal digital assistant devices and digital cameras become lighter and more compact in recent years, the color filters need to be lighter and thinner with higher color saturation as well. Therefore, it is crucial to increase the concentration of the coloring agent in the coloring composition. Nevertheless, as the concentration of the coloring agent is increased, the resin in photosensitive coloring composition will be relatively reduced. When the resin component on the pixel surface that contributes to the adhesion to the protective film is reduced, the adhesion between the pixel and the protective film is reduced, such that the protective film peels off from the interface with the pixel easily, and as a result, the high precision pattern linearity is poor.
In general, in the photosensitive resin composition of the aforementioned usage purpose, multi-functional photocurable monomers with polymerizable unsaturated bond, alkali-soluble resin, and a composition thereof with a photoinitiator are used. For example, Japanese Patent No. S61-213213 and Japanese Patent No. H1-152449 exemplify an application of the material for the color filter and disclose a copolymer of a (meth)acrylic acid or a (meth)acrylate with a carboxyl group, a maleic anhydride and other polymerizable monomers which is used as the alkali-soluble resin.
However, the copolymer disclosed here is a random copolymer and thus, generates a distribution of alkali dissolution rate inside a light-irradiated and a light-unirradiated parts, and as a result, it is difficult to obtain good developability or linearity in the high precision pattern during the development operation.
Although Japanese Patent No. H5-070528 refers to that a alkali-soluble photosensitive resin can be formed by reacting an epoxy acrylate compound having fluorene ring and an anhydride in molecules, and thereby, though the developability can be improved, the defects of poor contour angle in high precision still exist.
Therefore, how to improve the developability, high precision pattern linearity, and contour angle to meet the requirements of the industry is a goal of research in the technical field of the invention.